Measuring flow-polymer misalignment fluctuations that drive instability

Viscoelastic flow instabilities limit polymer processing rates. Spatially and time-resolved measurements of stress and flow, and calculations of the same, may help clarify the mechanisms of hydrodynamic fluctuations and instability. We therefore studied flow of high-molar-mass polyethylene oxide solutions through a cross-slot geometry. At sufficiently high speeds, flow switches its asymmetric flow direction aperiodically. Data was acquired by synchronized velocimetry (2D piv and 3D holographic tracking) and stress measurements (polarization imaging) at sub-ms resolution. 3D numerical simulations (of the Giesekus model) also demonstrate similar flow switching behavior and confirm buildup of flow-polymer misalignment prior to switching of flow asymmetry. Extending this work to explore how molecular characteristics, fluid composition, and channel design might moderate susceptibility to misalignment may lead to more efficient processing.